Build the Frame

1. Drill a 5/16-inch hole in the center of the 2-inch side of each leg, 3 inches from one end.

2. On the narrow side of two legs, mark 12 inches from the end opposite the holes. Align the bottom of a plywood piece with the marks, hold the outer edges of the plywood and the legs flush, and drill three ?-inch pilot holes on each side, ? inch from the edge. Screw the plywood to the legs. Mark the legs 6 inches from the top of the plywood. Line up the bottom edge of the remaining piece of plywood on the marks and fasten it to the legs. This is the launcher's front.

3. Lay the other two legs on a level surface, parallel and with the outer edges 46½ inches apart. Mark the narrow side of each leg 18 inches from the bottom, line up the crossbeam with the marks, and fasten it with one screw on each side. This is the back.

4. Connect the two assemblies by fitting the narrower one inside the other, lining up the 5/16-inch holes and putting the eyebolts through the holes, with the eye on the inside. Use three washers per side, one each between the eye and the inside leg, the two legs, and the wingnut and the outer leg.

5. Screw an eye lagbolt into each leg, 36 inches from bottom. Use the carabiners and chains to connect the eyebolts.

6. Drill two ½-inch holes opposite each other into funnel, 1 inch from top; fit grommets into holes. Slide 6-inch piece of hose onto rope; push rope's ends through narrow end of funnel and hole in fender washer. Tie the ends together.

7. Tie two 24-inch lengths of surgical tubing through the grommets and the eyebolts. Stand behind the frame, grasp the balloon holder's handle, and pull down at an angle. Load the funnel with a water balloon—and let it fly.